72 METABOLISM 



but that the presence of reducing substances in the medium prevents its direct access to 

 the organisms ; or the inhibiting effect may be due to the formation of oxidation products 

 through the mediation of catalysts which are inactivated in cultures at Eh levels per- 

 mitting the growth of obligate anaerobes. The change over from aerobic to anaerobic 

 growth may in fact be brought about by artificial inactivation of enzyme systems. Broh- 

 Kahn and Mirsky (1938), by cyanide-inhibition of enzymes capable of reacting with 

 molecular oxygen, induced Bad. coli to break down glucose anaerobically in the presence 

 of air, and it is obvious that similar inhibitions may take place as a result of changes 

 in metabolic processes that are reflected in the lower Eh levels of anaerobic cultures. 

 The nature of the hypothetical inhibitory oxidation products is still in doubt. It has 

 long been known that certain bacteria are sensitive to hydrogen peroxide (Traugott 1893, 

 Freer and Novy 1902). This sensitivity is associated with absence of catalase production. 

 The pneumococcus, for instance, produces HgOj, to which it is sensitive, and no catalase 

 (McLeod and Gordon 1922, Avery and Morgan 1924, Avery and Neill 1924a, b, c, Neill 

 and Avery 1925). Other organisms, like Sh. shigce, are moderately sensitive, but produce 

 neither H2O2 nor catalase. Most bacterial species produce catalase more or less actively 

 (see Gottstein 1893, Lowenstein 1903, Rywosch and Rywosch 1907). The fact that 

 anaerobes produce no catalase led McLeod and Gordon (1925a, b) to suggest that the 

 sensitivity of obligate anaerobes to oxygen is due to their readiness to form in its presence 

 inhibitory concentrations of HgOj. The acceptance of this hypothesis depends on the 

 demonstration of oxygen utilization and HjOj production by the anaerobes in the presence 

 of oxygen. McLeod and Gordon grew the anaerobes in blood media, and inferred the 

 production of HgOg from certain colour changes in the medium. Similar changes, however, 

 can be produced by reducing systems in the absence of air (Anderson and Hart 1934) and 

 do not necessarily indicate HgOj. Moreover, Cook and Stephenson (1928) were unable 

 to demonstrate oxygen utilization by CI. sporogenes in conditions that would have detected 

 the oxygen uptake equivalent to the formation of 1 : 50,000 H2O2 in their suspensions. 



Other Factors Influencing the Growth of Bacteria. 



Hydrogen Ion Concentration. — For any given species of bacterium there is an optimal, 

 and relatively narrow range of pH allowing vigorous growth, and a wider range extending 

 on each side of the optimum over which growth occurs less vigorously. For most of the 

 bacteria with which we are concerned the optimal pH lies a little to the alkaUne side 

 of neutrality (pH 7-2-7-6). The range of pH over which growth is possible has not been 

 accurately determined for many bacterial species, but for most pathogenic species it 

 would appear to extend over some such range as pH 50 to pH 80 (see Chapter 9). 



There are, however, species that show very distinctive variations from this modal 

 range of sensitivity. Azotobacter, for instance, is very sensitive to acids, and will not 

 grow in pure culture at a pH lower than 6-5 (Fred and Davenport 1918). The aciduric 

 bacilli of the genus Lactobacillus are, on the other hand, highly resistant to acids, and 

 will apparently grow to some extent at pH 4-0 or even less ; though the power to grow 

 has not, in this instance, been decisively distinguished from the power to resist the lethal 

 action of the acid (Mcintosh et al. 1922, 1924). The cholera vibrio is very tolerant of 

 alkali, relatively sensitive to acid. Its optimum for growth is pH 7-6-8-0 and its limits 

 for growth about pH 6-4-9-6. The enterococcus affords a good example of an organism 

 with a wide growth-range, extending from pH 4-8 to pH 110 (Downie and Cruickshank 

 1928, Davis and Thiel 1939). 



Temperature. — We need only note in this section that, for each species of bacterium, 

 there is an optimal temperature for growth and a range of temperature over which growth 

 is possible. For most pathogenic bacteria the optimal temperature for growth is in the 

 neighbourhood of 37° C, and the range of temperature over which growth occurs is 

 approximately 15°-40° C. Here as elsewhere, however, there are wide variations, especially 

 when we include in our survey non-pathogens as well as pathogens. In their reactions 

 to changes in temperatiu-e, bacteria as a whole display in a striking fashion their capacity 

 for adaptation to a wide range of environmental conditions. 



